PTC composition and PTC device comprising it

ABSTRACT

The present invention relates to a conductive polymer composition which exhibits PTC characteristics (PTC composition) and a PTC device comprising it, wherein the PTC composition comprises: a) at least one crystalline olefin-based polymer and at least one ionomer; and, b) electrically conductive particles which have been dispersed in polymer matrix formed by a). The PTC composition according to the present invention has an enhanced adhesion to electrodes, which minimizes contact resistance and increases the maximum working current (hold current). The PTC device comprising the PTC composition can be used as a circuit protection device which protects circuit from overflowing and which holds initial resistance value although it is reused after multiple short circuit have taken place.

TECHNICAL FIELD

The present invention relates to a conductive polymer composition havingPTC (positive temperature coefficient) characteristics (that is, PTCcomposition) and to a PTC device using the same.

BACKGROUND ART

A conductive material exhibiting a resistance change according to atemperature change and a device using the same have been well known. Aconventional PTC resistor has been known as a PTC thermistor using adoped BaTiO₃ ceramic material. A thermistor made of the ceramic materialexhibits a sharp PTC resistance effect at a higher temperature than itsCurie temperature. Although the PTC device made of the ceramic materialhas long been used, it has a problem that it is restricted inapplications and causes a high process expense because it has arelatively high resistance value at room temperature.

In an effort to solve the above problem, a conductive polymercomposition that can be more easily fabricated compared to theconventional ceramic process, as well as which has a small resistancevalue at room temperature, has been developed. As examples, U.S. Pat.No. 4,237,441, U.S. Pat. No. 4,545,926 and U.S. Pat. No. 5,880,668 aregiven.

The conductive polymer compositions disclosed in the above documentsexhibit “PTC characteristics” in which it has an electrical conductivityby uniformly dispersing carbon black or metal as a conductive fillerinto a polymer matrix, whereby its resistance is increased in proportionto a temperature rise, and its resistance is rapidly increased when thetemperature goes up to higher than a certain point called a switchingtemperature.

The polymers used for the conventional PTC composition are mostlyolefin-based polymers, for example, polyethylene (PE), polypropylene(PP), ethylene/propylene co-polymers and ethylene-based co-polymers suchas ethylene/(meta)acrylic acid co-polymers, ethylene/ethyl acrylateco-polymers, ethylene/butyl acrylate co-polymers and ethylene/vinylacetate co-polymers. Besides, polyvinyl-based co-polymers such aspolyvinylchloride, polyvinylidenechloride, polyvinylfluoride,polyvinylidenfluoride, thermoplastic polymers such as polyamide,polystyrene, polyacrylonitrile, silicone resins, polyester, a modifiedcellulose or polysulfone may be used.

The PTC composition is typically used as a circuit protection device forlimiting a current flow when a short-circuiting has taken place in thecircuit comprising a heater, a positive character thermistor, athermo-responsive sensor, a battery or the like, and for recovering thecircuit to a normal state when the cause of the short-circuiting isremoved. In addition, as an example of using the PTC composition, a PTCdevice, in which more than two electrodes are electrically connected tothe PTC composition, can be given. The electrodes are connected to apower supply so that the current can flow through the PTC component. ThePTC device is used as a protecting device for a circuit from currentoverload, overheating and the like, by functioning as a self-temperaturecontroller as described above.

The device generally allows current to flow through a circuit since theresistance is low enough at a temperature below the switchingtemperature (Ts). However, at a temperature above the switchingtemperature, it does not allow any further current to flow, by rapidlyincreasing the resistance. In other words, when the circuit is heated toa critical temperature, the PTC device functions as a circuit protectingdevice for decreasing a current overload caused by a short-circuiting toa lower and stable value. When the cause of the short-circuiting isremoved, the PTC device is cooled down below the critical temperatureand returned to the low resistance state of its normal operation. Sucheffect is called a “reset”. The composition of which the PTC device isconstructed is necessary to have such a current limiting performance andreset property allowing a repeated use at high voltage.

A polymer PTC electric circuit protecting device is generally formed byinserting a PTC component, which is fabricated by dispersingelectrically conductive fine particles such as of a metal or carbonblack into polymers, between a pair of electrodes. The electrodes areconnected to a power supply so that the current can flow through the PTCcomponent. In order to minimize a contact resistance, the electrodes aregenerally attached to the PTC composition by a thermo-fusion. However,in such methods, adhesion between components in the composition has beena problem. In order to overcome the problem, in the past, the surface ofthe electrodes was chemically or physically treated to be rough, orspecially fabricated electrodes have been used (Japanese Laid OpenPublication No. 5-109502 and U.S. Pat. No. 3,351,882, etc.). However,those methods have disadvantages in that the problem of contactresistance is not satisfactorily solved, and it is difficult to expectthe repetition stability returning to the same resistance value as thatof the initial stage even after several times of short-circuiting havetaken place.

In addition, when a high working current is required even though Itssize is limited such as in a lithium ion battery, the PTC device to beinserted into the circuit is also limited in size. In general, in caseof a PTC device, the maximum current value (that is, a hold current,I_(Hmax)), which is maintained at a normal working state withoutswitching, differs according to the power consumption. The powerconsumption is related to an initial resistance of the device. The lowerthe initial resistance is, relatively the less the power consumption is,and accordingly, the PTC device can have a high maximum hold current.Thus, in the PTC device, as it has a high maximum hold current, in orderto lower the resistance value of the device, the distance between a pairof electrodes is made short or the surface area of the electrodes has tobe enlarged. If the space between the two electrodes becomes narrow, theresistance value of the device is lowered down as much. However, if thespace between the electrodes is too narrow, a PTC component constructedtherebetween may easily be cracked by even a Weak external impact, andit is not easy to manufacture, too. Therefore, in general, the area ofthe electrodes is enlarged while maintaining a certain thickness. Inthis respect, if the resistance value of the PTC component insertedbetween the electrodes is not low enough, the size of the formed deviceshould be inevitably enlarged to larger than the limited circuit size tohave a high hold current. In addition, if the contact resistance is highdue to an insufficient adhesion, power consumption may be concentratedin the interface of the electrodes and the PTC component, andaccordingly it is impossible to obtain the high maximum hold current.

In other words, the resistance value of the PTC component itself and thecontact resistance between the electrodes and the PTC component shouldbe low enough so as to retain a high hold current while allowing the PTCdevice to be inserted into a limited size of circuit to have asufficiently small size.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to solve problems ofconventional device, and to provide a PTC composition that exhibits alow resistance and a favorable electrical conductivity when a commoncurrent flows in a circuit. In particular, the PTC composition of thepresent invention is capable of minimizing a contact resistance byimproving an adhesion in the interface of the electrodes and the PTCcomposition without any special treatment to the electrodes and iscapable of having a high maximum hold current.

Another object of the present invention is to provide a circuitprotecting device that is capable of maintaining an initial resistancevalue repeatedly and stably even in passing a current due to severaltimes of short-circuiting.

The above and other objects described in the detailed description of theinvention are achieved by providing a PTC composition comprising, a) acrystalline olefin-based polymer and an ionomer and b) conductiveparticles dispersed in a polymer matrix formed with a).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing PTC characteristics of a PTC compositionobtained by Examples 1 through 4.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a PTC composition which has a lowresistance and a favorable electrical conductivity when a common currentflows, and which is, in particular, capable of minimizing a contactresistance by improving an interfacial adhesion between electrodes andthe PTC composition without any special treatment to the electrodes andwhich has a substantially high hold current even in a small size. Inparticular, the present invention relates to a PTC compositioncomprising, a) a crystalline olefin-based polymer and an ionomer and b)conductive particles dispersed in a polymer matrix formed with a).

The crystallinity of the olefin-based polymer used for the PTCcomposition of the present invention is to be at least 10%, and,preferably, at least 20%, and, more preferably, at least 40%. Thecontent of the olefin-based polymer is adjusted to be at least 60% byweight and, preferably, in the range of 60-99.5% by weight of the totalpolymer weight in the PTC composition.

The olefin-based polymer is preferably selected from the groupconsisting of polyethylene (PE), polypropylene (PP), a co-polymer ofethylene and a monomer having a polar group, a co-polymer of propyleneand a monomer having a polar group and mixtures thereof.

Examples of the polyethylene include a high-density polyethylene (HDPE),a middle-density polyethylene (MDPE) and a low-density polyethylene(LDPE), a linear low-density polyethylene (LLDPE) and mixtures thereof,of which the high-density polyethylene is more preferable.

Examples of the co-polymer of ethylene or propylene with a monomerhaving a polar group include ethylene/acrylic acid co-polymers,ethylene/methacrylic acid co-polymers, ethylene/ethyl acrylateco-polymers, ethylene-butylacrylate co-polymers, ethylene-vinylacetateco-polymers, ethylene-itaconic acid co-polymers, ethylene/monomethylmalate co-polymers, ethylene/maleic acid co-polymers, ethylene/acrylicacid/methylmethacrylate co-polymers, ethylene/methacrylic acid/ethylacrylate co-polymers, ethylene/monomethyl malate/ethyl acrylateco-polymers, ethylene/methacrylic acid/vinylacetate co-polymers,ethylene/acrylic acid/vinylalcohol co-polymers,ethylene/propylene/acrylic acid co-polymers, ethylene/styrene/acrylicacid co-polymers, ethylene/methacrylic acid/acrylonitril co-polymers,ethylene/fumaric acid/vinyl methyl ether co-polymers,ethylene/vinylchloride/acrylic acid co-polymers, ethylene/vinylidenechloride/acrylic acid co-polymers, ethylene/trifluoroethylenechloride/methacrylic acid co-polymers and corresponding propyleneco-polymers.

Maleic anhydride-grafted polyethylene, and more specifically, maleicanhydride-grafted high-density polyethylene (m-HDPE) or maleicanhydride-grafted low-density polyethylene (m-LDPE), etc. can also beused for the PTC composition of the present invention.

Preferably, the olefin-based polymer is selected from the groupconsisting of high-density polyethylene, maleic anhydride-graftedhigh-density polyethylene, ethylene/ethyl acrylate co-polymers,ethylene/vinylacetate co-polymers and mixtures thereof.

Examples of the ionomer used for the PTC composition of the presentinvention together with the olefin-based polymer include anethylene-based ionomer, a styrene-based ionomer, a rubber-based ionomer,a fluorine-based ionomer, a sulfonated EPDM, a carboxylated nitril-basedionomer and combinations thereof. Preferably, the ionomer is selectedfrom the group consisting of ethylene-based ionomers, styrene-basedionomers, rubber-based ionomers and combinations thereof.

Preferably, the ethylene-based ionomer has the following formula (I):

In formula (I), R represents hydrogen or methyl; X⁻ represents COO⁻ orC₆H₄SO₃ ⁻; and M⁺ represents a metallic ion or an ammonium ionsubstituted with an alkyl or aryl group.

If the M⁺ is a metallic ion, it is preferably selected from the groupconsisting of monovalent, divalent and trivalent metallic ions includingLi⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Ba²⁺, Zn²⁺ and Al³⁺. If M⁺ is an ammonium ionsubstituted with an alkyl or aryl group, it is preferably selected fromthe group consisting of tetrabutylammonium ion andcetyltrimethylammonium ion.

Examples of the ionomer having the formula (I) include a co-polymer ofethylene/methacrylic acid sodium salt, a co-polymer of ethylene/acrylicacid zinc salt, and a co-polymer of ethylene/styrene sulfonic acidsodium salt. Those ionomers are sold in the market under the names ofSurlyn®, Aclyn®, Lucalen®, Lotek®, Escor®, etc.

In formula (I), the ratio of m/n is not necessarily limited, and it maybe no less than 0.1, more preferably, no less than 1, and mostpreferably, no less than 5.0.

Examples of the styrene-based ionomer include styrene/sodium(metha)acrylate co-polymers and styrene/butyl methacrylate/zinc acrylateco-polymers.

Examples of the rubber-based ionomer include butadiene/lithiummethacrylate co-polymers and butadiene/sodium acrylate co-polymers. Suchionomers are sold in the market under the name of Hycar®.

As a fluoride-based ionomer, the material sold currently under the nameof Nafion® or Flemion® may be used.

The amount of the ionomer used is not necessarily limited, but it ispreferred to be added 0.5-40% by weight and more preferably, 2-20% byweight to the total polymer content. The PTC composition containing theionomer according to the present invention has a unique physicalcross-linking structure due to the ionomer in the polymer composition.Such structure seems to be made by the ionic bond formed between ametallic ion and acid group of the polymer.

Besides the olefin-based polymer and ionomer, the PTC composition of thepresent invention may additionally contain a polyvinyl-based polymersuch as polyvinylchloride, polyvinylidenechloride, polyvinylfluoride andpolyvinylidenfluoride; polyamide; polystyrene; polyacrylonitril;silicone resin; polyester; modified cellulose; and thermoplastic polymermaterial such as polysulfone. When added, the content may be adjusted inthe range of 0.5-50% by weight of the total polymer content.

The conductive particles dispersed in the polymer matrix are used togive conductivity to the PTC composition of the present invention. Thetypes of usable conductive particles are not necessarily limited as longas they are typical conductive particles used for the PTC composition.Examples include powders of metal such as nickel, silver, gold, copperor metal alloys, metal-coated particles, carbon black and acetyleneblack.

The amount of the conductive particles used differs depending on thetype of the substance to be used. It is preferably 5-70% by weight ofthe total PTC composition.

Preferred conductive particles are carbon black. The average particlesize of carbon black is preferably at least 60nm with uniform particlesize distribution. Specific examples of carbon black which can be usedfor the present invention include Conductex 975, Raven 420, Raven 430,N660 of Columbian Chemicals Co., and Black Pearl 120, Black Pearl 130,Black Pearl 160, Vulcan XC72, etc. of Cabot Co., but are not limitedthereto.

The conductive particles have a different dispersion mechanism withinthe polymer composition according to the polarity of the polymer used.As the polarity is higher, the mutual force between the conductiveparticles and polymer resins is strengthened, and therefore, the bondingbetween the polymer resin and the carbon particles become stronger. Dueto this effect, the form of the dispersion of the conductive particlesin the polymer resin composition has a different characteristic. Thatis, the distribution of the conductive particles is influenced by thedistribution structure of the ionomer having the polar group used in thePTC composition of the present invention. As a result, an electronicpassageway can be easily made relatively in the PTC composition of thepresent invention, and these are the features of the present invention.Accordingly, even if the same conductive particles as in a conventionalPTC composition are used, unlike the conventional PTC composition, thePTC composition of the present invention has a lower resistance, andtherefore, the maximum hold current can be increased. In addition, theinteraction between the conductive particles and the polymers isconstant regardless of a temperature rise, so that change of theparticles' position or aggregation of particles within the polymermatrix can be restrained. Accordingly, when cooled, the matrix isshrunk, and thereby the initial state of distribution is maintained.Therefore, a restoration stability, by which the resistance is restoredto its initial value when the normal working state is restored after theresistance is much increased when the temperature goes up due to acurrent overload in the device, can be considerably increased. Additionof the ionomer having a polar group to the PTC composition is helpful toincrease the adhesion between electrodes and the PTC composition, andbesides, it contributes to the enhancement of the stability of the PTCdevice.

The PTC composition of the present invention may further contain aco-processing agent not influencing on the properties of thecomposition, such as an antioxidant, an anti-degradation agent, ananti-foaming agent and the like.

The PTC device of the present invention is constructed in the followingmanner. Conductive particles, preferably, a carbon black, and anantioxidant are added to a mixture of an olefin-based polymer andionomer, the resulting mixture is blended using a Bravendar, Bnbari,homo-mixer or the like, and then at least one metallic electrode isshaped to the obtained conductive polymer composition. After shaping, inorder to improve stability and reliability of the device, the obtainedpolymer PTC composition is cross-linked by mainly using an electronbeam. At this time, according to the type, content and thickness of thecomposition, the electron beam is irradiated at a strength of 1-100Mrads, preferably, 5-50 Mrads. The shape of the electrode is determineddepending on the shape of the device. For example, there is a foil,wire, powder, paste or the like of a metal. In the present invention,two thin metallic films are attached onto both sides of the conductivepolymer composition, to be shaped as the plate-type polymer compositionis inserted between two electrodes. A lead electrode is shaped onto thetwo plate-shape electrodes so as to be connected to an electric circuit.A wire or plate of a metal is soldered at the lead electrode. Thematerial of the electrode may be a metal such as iron, copper, tin,nickel, silver or the like.

The circuit protecting device with such a shaped electrode usually has aresistance of below 5 Ω, preferably below 1 Ω, and more preferably below0.1 Ω, at room temperature (25° C.). When the temperature rises, at ahigher temperature than a critical temperature where the device isswitched, the maximum resistance value becomes at least 10³ Ω, andpreferably at least 10⁴ Ω.

EXAMPLES

The present invention will now be described clearly with the followingexamples, but the scope of the present invention is not limited thereto.

Example 1

90 parts of high-density polyethylene (HDPE), 10 parts of ionomer Surlyn7930, 100 parts of carbon black (Raven 420, Columbian Chemicals Co.) asa conductive material and 0.4 parts of antioxidant (Irganox 1010,Ciba-Geigy Co.) were mixed at a speed of 60 rpm at 170° C. for 20minutes by using a Bravendar mixer (Plasti-corder, PLE 331).

The mixed composition was put into a mold, pressed to make a thin plateof 0.5 mm in thickness under a pressure of 450 Kgf/cm² at 180° C. byusing a hydraulic press, set aside under a pressure of 110 Kgf/cm² at80° C. for one hour, and then allowed to return to room temperature andatmospheric pressure.

An electro-deposited copper foil of 30 μm in thickness having amicro-level of roughness on the surface of one side was melted andpressed to both sides of the plate of the conductive polymer compositionobtained as described above, thereby to shape plate-type electrodes.

The plate of the conductive polymer composition stacked with theplate-type electrodes was cross-linked at an irradiation strength of 20Mrads by using a particle beam accelerator and shaped in a disk typehaving a diameter of 12.7 mm by using a punch.

And then, the obtained device and tin-coated copper wire were put into asolvent which is used for removing oxide from a melted metal andpreventing additional oxidation of the melted metal and put again into amelted solder bath. The PTC device and the tin-coated copper wire werethen taken out from the solder bath, cooled, and the tin-coated copperwire was attached to the surface of the plate-type electrodes stackedonto the PTC device.

The electrical and PTC properties of the electric circuit protectingdevice fabricated as described above were measured by the followingprocedures, and the results are shown in Table 1.

(1) The device was set aside at a temperature above the melting point ofthe polymer composition used for the fabrication of the device for 10minutes, cooled to room temperature, and then its resistance wasmeasured. While the temperature around the device was gradually raisedat a rate of 2° C./min, the resistance change according to thetemperature change was measured with a digital multimeter (Keithley2000). The ratio between the initial and maximum resistance values wascalculated by using the resistance value change measured and indicatedas “PTC intensity”.

(2) After the PTC device was inserted into a circuit constructed formeasuring the maximum hold current, a stabilized current within thedevice was measured while gradually increasing an applied voltage bytaking 0.05 V as one step. The voltage was continuously increased untilthe device was completely switched. While increasing the appliedvoltage, the current value that passed through the PTC device wasmeasured, and the maximum current value was defined as a “maximum holdcurrent, I_(Hmax)”. If the voltage is increased over this point, thecurrent falls down.

Comparative Example 1

Instead of 90 parts of HDPE and 10 parts of ionomer Surlyn 7930 ofExample 1, 100 parts of HDPE was used to fabricate a PTC device in thesame manner as described in Example 1. Physical properties of the PTCdevice were measured, and the results are shown in Table 1.

Example 2

Instead of the 10 parts of ionomer Surlyn 7930 in Example 1, 95 parts ofmaleic anhydride-grafted HDPE (m-HDPE) and 5 parts of ionomer Surlyn8660 were used to fabricate a PTC device in the same manner as describedin Example 1. Physical properties of the PTC device were measured, andthe results are shown in Table 1.

Comparative Example 2

Instead of the 95 parts of maleic anhydride-grafted HDPE (m-HDPE) andthe 5 parts of Surlyn 8660 in Example 2, 100 parts of maleicanhydride-grafted HDPE (m-HDPE) was used to fabricate a PTC device inthe same manner as described in Example 1. Physical properties of thePTC device were measured, and the results are shown in Table 1.

Example 3

Instead of the 90 parts of HDPE and the 10 parts of ionomer Surlyn 7930in Example 1, 95 parts of maleic anhydride-grafted HDPE (m-HDPE) and 5parts of ionomer Aclyn 295 were used to fabricate a PTC device in thesame manner as described in Example 1. Physical properties of the PTCdevice were measured, and the results are shown in Table 1.

Example 4

Instead of the 90 parts of HDPE and the 10 parts of ionomer Surlyn 7930in Example 1, 85 parts of maleic anhydride-grafted HDPE (m-HDPE) and 15parts of ionomer Lotek 8020 were used to fabricate a PTC device in thesame manner as described in Example 1. Physical properties of the PTCdevice were measured, and the results are shown in Table 1.

Example 5

Instead of the 90 parts of HDPE and the 10 parts of ionomer Surlyn 7930in Example 1, 90 parts of maleic anhydride-grafted HDPE (m-HDPE), 5parts of ionomer Surlyn 7930 and 5 parts of Aclyn 285 were used tofabricate a PTC device in the same manner as described in Example 1.Physical properties of the PTC device were measured, and the results areshown in Table 1.

Example 6

Instead of the 90 parts of HDPE and the 10 parts of ionomer Surlyn 7930in Example 1, 90 parts of maleic anhydride-grafted HDPE (m-HDPE) and 10parts of ionomer styrene/sodium methacrylate co-polymer were used tofabricate a PTC device in the same manner as described in Example 1.Physical properties of the PTC device were measured, and the results areshown in Table 1.

Example 7

Instead of the 90 parts of HDPE and the 10 parts of ionomer Surlyn 7930in Example 1, 90 parts of maleic anhydride-grafted HDPE (m-HDPE) and 10parts of ionomer butadiene/sodium acrylate co-polymer were used tofabricate a PTC device in the same manner as described in Example 1.Physical properties of the PTC device were measured, and the results areshown in Table 1.

Example 8

Instead of the irradiation strength of 20 Mrads in Example 1, thecross-linking was carried out with an irradiation strength of 10 Mradsto fabricate a PTC device in the same manner as described in Example 1.Physical properties of the PTC device were measured, and the results areshown in Table 1.

Example 9

Instead of the carbon black (Raven 420, Columbian Chemicals Co.) as aconductive material in Example 1, a carbon black (Monarch 120, Cabot,Corp.) as a conductive material was used to fabricate a PTC device inthe same manner as described in Example 1. Physical properties of thePTC device were measured, and the results are shown in Table 1.

Example 10

Instead of the 90 parts of HDPE in Example 1, 80 parts of HDPE and 10parts of ethylene/ethyl acrylate co-polymer were used to fabricate a PTCdevice in the same manner as described in Example 1. Physical propertiesof the PTC device were measured, and the results are shown in Table 1.

Example 11

Instead of the 90 parts of HDPE of Example 1, 85 parts of maleicanhydride-grafted HDPE (m-HDPE) and 5 parts of ethylene/vinylacetateco-polymer were used to fabricate a PTC device in the same manner asdescribed in Example 1. Physical properties of the PTC device weremeasured, and the results are shown in Table 1.

Example 12

Instead of the 90 parts of HDPE and the 10 parts of ionomer Surlyn 7930in Example 1, 90 parts of maleic anhydride-grafted HDPE (m-HDPE), 5parts of polyvinylidenefluoride tri-co-polymer (Kynar 9301, Pennwalt)and 5 parts of ionomer Surlyn 7930 were used to fabricate a PTC devicein the same manner as described in Example 1. Physical properties of thePTC device were measured, and the results are shown in Table 1.

Example 13

Instead of the 90 parts of HDPE and the 10 parts of ionomer Surlyn 7930in Example 1, 85 parts of HDPE, 10 parts of Nylon-12 (Aeson-TL, ElfAtochem) and 5 parts of ionomer Surlyn 7930 were used to fabricate a PTCdevice in the same manner as described in Example 1. Physical propertiesof the PTC device were measured, and the results are shown in Table 1.

Example 14

PTC characteristics of the PTC compositions obtained in Examples 1through 13 and Comparative examples 1 to 3 were tested, and the resultsare shown in FIG. 1. As shown in FIG. 1, the composition of the presentinvention has excellent PTC characteristics. In more detail, FIG. 1shows that the PTC composition of the present invention has PTCcharacteristics in that it has a low initial resistance value so thatthe maximum hold current can be increased, its switching temperature isin the range of 100-120° C., and its resistance is sharply increased ata temperature above its switching temperature to be able to cut offcurrent flow.

TABLE 1 Physical Properties Resistance Change at room rate Initial tem-of resistance perature resistance at room after at room Maximum PTC tem-witching tem- hold Intensity perature 100 perature current (R_(max)/ (Ω)times (Ω) (%) (mA) R_(min)) Example 1 0.50 0.77 53 1837 1.8 × 10⁵Comparative 0.53 1.19 122 1451   9 × 10⁵ Example 1 Example 2 0.49 0.7248 2198 2.6 × 10⁴ Comparative 0.58 0.96 67 1821   2 × 10⁷ Example 2Example 3 0.50 0.66 32 2167 1.7 × 10⁶ Comparative 0.48 0.72 49 1431   4× 10⁵ Example 3 Example 4 0.49 0.64 29 2000   4 × 10⁴ Example 5 0.480.67 40 1925 3.8 × 10⁵ Example 6 0.47 0.63 34 2067 9.6 × 10⁴ Example 70.48 0.68 43 2032 3.6 × 10⁴ Example 8 0.50 0.84 68 1829 4.6 × 10⁴Example 9 0.60 0.91 51 1729 5.7 × 10⁵ Example 10 0.53 0.77 46 1802 8.2 ×10⁴ Example 11 0.62 0.96 55 1796 6.2 × 10⁴ Example 12 0.78 1.35 73 10524.5 × 10⁶ Example 13 0.65 1.01 55 2320 7.6 × 10⁴

Industrial Applicability

As so far described, the PTC composition of the present invention hasthe following advantages:

The interfacial adhesion between the PTC composition and electrodes canbe improved so that the contact resistance can be minimized even withouta special treatment to the electrodes, and accordingly, the maximum holdcurrent can be improved. As shown in Table 1, unlike the conventionalPTC composition, the PTC composition of the present invention has a lowinitial resistance value and is capable of increasing the maximum holdcurrent.

Accordingly, the PTC device comprising the PTC composition of thepresent invention can be applied as a circuit protecting device which isable to maintain the initial resistance value stably even though it isrepeatedly used after several times of short circuit have taken place.

What is claimed is:
 1. A conductive polymer composition having PTCcharacteristics comprising: a crystalline olefin-based polymer and anionomer; and conductive particles dispersed in a polymer matrix formedwith a); wherein the amount of the olefin-based polymer is in the rangeof 60-99.5% by weight of the total polymer content, the amount ofionomer is in the range of 0.5-40% by weight of the total polymercontent, and the content of the conductive particles is in the range of5-70% by weight of the total weight of the PTC composition.
 2. Thecomposition according to claim 1, wherein the crystallinity of theolefin-based polymer is no less than 20%.
 3. The composition accordingto claim 1, wherein the olefin-based polymer is selected from the groupconsisting of polyethylene, polypropylene, a co-polymer of ethylene witha monomer having a polar group, a co-polymer of propylene with a monomerhaving a polar group and mixtures thereof.
 4. The composition accordingto claim 1, wherein the olefin-based polymer is selected from the groupconsisting of a high-density polyethylene, a high-density maleicanhydride-grafted polyethylene, an ethylene/ethyl acrylate co-polymer,an ethylene/vinylacetate co-polymer and mixtures thereof.
 5. Thecomposition according to claim 1, wherein the ionomer is selected fromthe group consisting of an ethylene-based ionomer, a styrene-basedionomer, a rubber-based ionomer, a fluorine-based ionomer, a sulfonatedEPDM, a carboxylated nitril-based ionomer and mixtures thereof.
 6. Thecomposition according to claim 1, wherein the ionomer is selected fromthe group consisting of an ethylene-based ionomer, a styrene-basedionomer, a rubber-based ionomer and mixtures thereof.
 7. The compositionaccording to claim 1, wherein the ionomer is an ethylene-based ionomerhaving the following formula (I):

wherein, R represents hydrogen or methyl; X⁻ represents COO⁻ or C₆H₄SO₃⁻; M⁺ represents a metallic ion or an ammonium ion substituted with analkyl or aryl group; and m/n is no less than 0.1.
 8. The compositionaccording to claim 7, wherein the M⁺ is selected from the groupconsisting of monovalent, divalent and trivalent metallic ions includingLi⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Ba²⁺, Zn²⁺ and Al³⁺.
 9. The compositionaccording to claim 7, wherein the M⁺ is selected from the groupconsisting of tetrabutylammonium ion and cetyltrimethylammonium ion. 10.The composition according to claim 1, wherein the ionomer is astyrene-based ionomer selected from the group consisting ofstyrene/sodium (metha)acrylate co-polymers, styrene/butylmethacrylate/zinc acrylate co-polymers and mixtures thereof.
 11. Thecomposition according to claim 1, wherein the ionomer is selected fromthe group consisting of butadiene/lithium methacrylate co-polymers,butadiene/sodium acrylate co-polymers and mixtures thereof.
 12. Thecomposition according to claim 1, wherein the conductive particles usedfor the PTC composition are selected from the group consisting ofpowders of metals including nickel, silver, gold, copper and metalalloys, metal coated particles, carbon black and acetylene black. 13.The composition according to claim 1, wherein the conductive particlesused for the PTC composition is carbon black.
 14. The compositionaccording to claim 1, further comprising a thermoplastic polymerselected from the group consisting of a polyvinyl-based polymerincluding polyvinylchloride, polyvinylidenechloride, polyvinylfluorideand polyvinylidenefluoride; polyamide; polystyrene; polyacrylonitril;silicone resin; polyester; modified cellulose, polysulfone and mixturesthereof.
 15. The composition according to claim 1, further comprising anantioxidant, an anti-degradation agent and/or an anti-foaming agent. 16.A circuit protecting PTC device in which at least two thin metallicfilms are attached onto both sides of the conductive polymer compositionhaving PTC characteristics according to claim 1, thereby to connect theelectrodes electrically.
 17. A circuit comprising the circuit protectingPTC device according to claim
 16. 18. The composition according to claim3, wherein the crystallinity of the olefin-based polymer is no less than20%, and wherein the ionomer is selected from the group consisting of anethylene-based ionomer, a styrene-based ionomer, a rubber-based ionomer,a fluorine-based ionomer, a sulfonated EPDM, a carboxylated nitril-basedionomer and mixtures thereof, and wherein the conductive particles usedfor the PCT composition are selected from the group consisting ofpowders of metals including nickel, silver, gold, copper and metalalloys, metal coated particles, carbon black and acetylene black. 19.The composition according to claim 18, wherein the olefin-based polymeris selected from the group consisting of a high-density polyethylene, ahigh-density maleic anhydride-grafted polyethylene, an ethylene/ethylacrylate co-polymer, an ethylene/ethyl acrylate co-polymer and mixturesthereof, and wherein the ionomer is selected from the group consistingof an ethylene-based ionomer, a styrene-based ionomer, a rubber-basedionomer and mixtures thereof.
 20. A circuit protecting PTC device inwhich at least two thin metallic films are attached onto both sides ofthe conductive polymer composition having PTC characteristics accordingto claim 19, thereby to connect the electrodes electrically.